Context.Optical long-baseline interferometry is moving a crucial step forward
with the advent of general-user scientific instruments that equip
large aperture and hectometric baseline facilities, such as the Very
Large Telescope Interferometer (VLTI).

Aims.AMBER is one of the VLTI instruments that combines up to three beams
with low, moderate and high spectral resolutions in order to provide
milli-arcsecond spatial resolution for compact astrophysical sources
in the near-infrared wavelength domain. Its main specifications are
based on three key programs on young stellar objects, active
galactic nuclei central regions, masses, and spectra of hot
extra-solar planets.

Methods.These key science goals led to scientific specifications, which were
used to propose and then validate the instrument concept. AMBER uses
single-mode fibers to filter the entrance signal and to reach highly
accurate, multiaxial three-beam combination, yielding three
baselines and a closure phase, three spectral dispersive elements,
and specific self-calibration procedures.

Results.The AMBER measurements yield spectrally dispersed calibrated
visibilities, color-differential complex visibilities, and a closure
phase allows astronomers to contemplate rudimentary imaging and
highly accurate visibility and phase differential measurements.
AMBER was installed in 2004 at the Paranal Observatory. We describe
here the present implementation of the instrument in the
configuration with which the astronomical community can access it.

Conclusions.After two years of commissioning tests and preliminary observations,
AMBER has produced its first refereed publications, allowing
assessment of its scientific potential.

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